Electronic – the bode plot of an inverting op amp if you replace the resistors with caps

bode plotcapacitoroperational-amplifiertransfer function

What is the bode plot of an inverting op amp if you replace the resistors with caps? What if you now add a capacitive load to the output?

Update:

I ran a quick simulation and the bode plot looks as follows. It looks as though there are two poles but I am still confused about how to analytically derive the transfer function and analytically calculate the poles. Would it just be -C1/Cf where the frequency terms (jw) cancel out? enter image description here

Update 2:

It seems some clarification is needed on this question. A non-inverting op amp is shown below. In this common configuration, I want to analyze the circuit if you were to replace the resistors with capacitors. I ran a quick simulation as seen in my first update. I now understand that the gain of the circuit is -C1/Cf. However I am confused as to how to hand calculate poles for the bode plot of this circuit. Any help is much appreciated.

enter image description here

Best Answer

I understood the question as being a standard text book inverting opamp, with R1 and R2 replaced by capacitors C1 and C2.

The resistors become complex (imaginary) impedances Z1=1/jwC1 forward and Z2=1/jwC2 feedback. The TF is the ratio -C2/C1, as already mentioned by the OP.

(I do suggest the OP update the diagram to show the intended circuit.)

I assume this is homework, and thus the OpAmp is ideal. The TF is then a constant and independent of f.

There are many tutorials, in writing and on youtube, to derive the TF of such an arrangement. It's not complicated. One example is https://masteringelectronicsdesign.com/how-to-derive-the-inverting-amplifier-transfer-function/

Here is an example to plot the TF, from http://sim.okawa-denshi.jp/en/opampkeisan.htm

Note the capacitance is in the uF range, and the resistors are near zero (calculator will not accept R=0)

enter image description here

and the plots are flat well into the GHz range but not beyond that, due to the non-zero R.

enter image description here

In reality the TF will not be constant for f, as the capacitors and the OpAmps internal resistances will interact to form low- or high-pass filters.

Also, in practice, the capacitors and inductance of the op-amp leads will interact to form resonant circuits.

So this flatness only persists up to a certain frequency, and then it becomes a battle of the parasitics.